Add ext_transfer call (#5169)

* contracts: Add ext_transfer call

This call allows contracts to send balance to any account
contract or not. Previously, the only way to do that was
though ext_call.

* Apply suggestions from code review

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* The define_env! macro does not allow for trailing comma

* Update frame/contracts/src/exec.rs

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* Bump spec version

* Do not use nested gas meter

* Use explicit 0 or 1 as return value

* Remove superflous intermediate binding

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/bin/node/runtime/src/lib.rs

@@ -82,7 +82,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to 0. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 236,
+	spec_version: 237,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 };